A wide range of devices may be equipped for communication via a wireless local area network (WLAN), for example a PDA (personal digital assistant), an MP3 player, a bio-sensing device for heart-rate or blood pressure monitoring, an electronic newspaper, or devices for remote control applications. Such devices usually have a small form factor and therefore require an antenna that is compact.
Furthermore, such devices are commonly required to operate in more than one frequency band, for example at about 2.5 GHz and about 5.5 GHz, which places additional constraints on the antenna. An example of a technical standard for WLAN operation is IEEE802.11a/b/g, which is commonly referred to as WiFi.
WO2005/117205 A1 discloses a slot antenna comprising a conductive plane having a slot with an open end and a closed end and a connection point located near the slot more closely to the closed end than to the open end. The perimeter of the conductive plane is between 50% and 200% of the wavelength of operation. The antenna does not need to operate against a ground surface. Such an antenna is, however, suited to operation in only one frequency band.
Chih-Ming Su et al in “Dual-band Slot Antenna for 2.4/5.2 GHZ WLAN Operation”, Microwave and Optical Technology Letters, vol. 35, no. 4, pp 306-308, 20 Nov. 2002, disclose an antenna suited to dual-band operation. The antenna comprises a copper plate placed in contact with one side edge of a ground plane, and has two narrow linear slots, a longer one for operation at 2.45 GHz aligned in parallel with a shorter one for operation at 5.25 GHz.
There remains a requirement for compact and efficient dual-band antennas.